MFC r342379, r342383:
gai_strerror() - Update string error messages according to RFC 3493.

Error messages in gai_strerror(3) vary largely among OSs.

For new software we largely replaced the obsoleted EAI_NONAME and
with EAI_NODATA but we never updated the corresponding message to better
match the intended use. We also have references to ai_flags and ai_family
which are not very descriptive for non-developer end users.

Bring new error messages based on informational RFC 3493, which has
obsoleted RFC 2553, and make them consistent among the header and
manpage.